The present invention relates to color framers, and more particularly, to those that are usable with both NTSC and PAL signals.
During video editing, a capstan servo is used to ensure that synchronization exists between a video tape being played back and the video input of the VTR. This works well for monochrome, but for color signals it is necessary to also synchronize the phase relationship of the color burst zero crossings to the horizontal synchronization pulse for the played back and reference video signals.
This relationship for an NTSC signal is given below in Table I, where + indicates a positive going color burst zero crossing with respect to synchronization pulse relationship, and - indicates a negative going such relationship. The color burst and the horizontal synchronization pulse do not occur at the same time. However, since the color burst starts an integer number of cycles from the start of the horizontal synchronization pulse, this relationship is the same as if they occurred at the same time.
TABLE I ______________________________________ (NTSC) Line Frame 1 2 3 4 ______________________________________ 1 + - + - 2 - + - + 3 + - + - 4 - + - + 5 + - + - ______________________________________
In the above table all relationships are for field one of the frame in question. It will be noted that the phase relationship can only be off, if it is off at all, by only one frame. It is necessary to sample only one line per frame to determine enough information to achieve proper framing.
Table II below shows the same kind of relationship for field one of the frame in question for a PAL signal.
TABLE II ______________________________________ Line Frame 1 2 3 4 ______________________________________ 1 + + - - 2 + - - + 3 - - + + 4 - + + - 5 + + - - ______________________________________
The sequence above repeats after the fourth line, e.g. line 5 is the same as line 1 of the same frame and field, etc. It will be noted however, that the phase relationship can be off up to a maximum of three frames, e.g. if capstan lock up occurs on frame 2 when it should have been frame 1, then it will be necessary to go to frame 5 to obtain the same phase relationship for proper frame lock up. Further it is necessary to sample two lines per frame to obtain enough information to achieve proper framing.
Up till now, if it was desired to use a single framing circuit unit for both NTSC & PAL, it was necesssary to replace a module in the unit when switching from one standard to the other one due to the above described differences.
It is therefore desired to have a framer that can be used for both PAL and NTSC without changing modules.